The present invention relates to an improved shutter driving apparatus which uses a stepping motor to open and close shutter sectors.
Recent compact cameras are usually equipped with a shutter which uses a stepping motor to open and close the shutter sectors. As described in Japanese Laid-open Patent Publ. No. 60-57327, for example, an exposure program using a stepping motor is advantageous in that a high precision exposure can be effected under digital control and a shutter charging mechanism can be omitted.
If a flash unit is used in daylight, or if the depth of focus is widened, it is desirable to control the exposure to provide a small aperture diameter. Such exposure control may be effected by the exposure program using a stepping motor, by changing the electric driving mode of the stepping motor. In particular, the stepping motor is temporarily stopped when the shutter sectors have reached a position corresponding to a desired aperture size, and after passage of a certain time interval for exposure of the film, the stepping motor is rotated in a reverse direction to close the shutter sectors.
During exposure control at a desired aperture diameter, however, a so-called ripple motion of the shutter sectors often occurs. This ripple motion of the shutter sectors occurs when the stepping motor is stopped following high speed rotation. Specifically, inertia of the rotor of the stepping motor, inertia of a mechanism coupling the motion of the rotor to the shutter sectors and other associated inertia causes the rotor of the stepping motor to surpass, or overshoot the desired stop position, whereupon the stepping motor undergoes an attenuating motion to correct the shutter blades to the desired stop position.
This ripple motion of the sectors can occur during a so-called triangular exposure wherein the shutter sectors are closed prior to their being driven to a maximum open position. In this case, however, the stepping motor is rotated immediately thereafter in a reverse direction to close the sectors; also, during opening of the sectors to the maximum desired open position, a stopper limits the extent of opening of the sectors. Therefore, ripple motion during a triangular exposure is not a substantial problem. However, in exposure control at small diameter apertures, for example, in aperture priority exposure control, the sectors are usually stopped before reaching their maximum desired open position and held in place for a predetermined time. In this case, ripple motion of the sectors is therefore significant, and a large exposure error results.